Subscriber line interface circuits (SLICs) are employed by telecommunication service providers to interface a wireline pair with subscriber (voice—data) communication equipment. In order to be interfaced with different types of telecommunication circuits, including (single supply-based) low voltage circuits that provide digital codec functionality, the transmission channels of the SLIC must conform with a very demanding set of performance requirements, such as accuracy, linearity, is low noise, filtering, insensitivity to common mode signals, low power consumption, and ease of impedance matching programmability. In a typical application, the wireline pair to which the SLIC is connected can vary from one installation to another, and may have a significant length (e.g., on the order of multiple miles), transporting both substantial DC voltages, as well as AC signals (e.g., voice and/or ringing). As a result, it has been difficult to realize a SLIC implementation having ‘universal’ use in both legacy and state of the art installations.
Advantageously, this problem is successfully addressed by the SLIC architecture disclosed in the '976 application, referenced above and a portion of which is diagrammatically illustrated in FIG. 1. As shown therein, the SLIC of the '976 application is partitioned into a high voltage analog section 1, that drives tip and ring conductors 2, 3 of a subscriber loop pair 4, and a mixed signal (low voltage and digital signal processing (DSP)) section 5, which monitors and controls the operation of the high voltage analog section 1. The high voltage analog section 1 is comprised of an integrated arrangement of functional analog signal blocks, and is interfaced with a DSP codec subsection 6 and a supervisory microcontroller subsection 7 of the mixed signal section 5. The high voltage analog section 1 performs analog (e.g., voice, ringing) signal processing and interface functions of a conventional SLIC, based on control inputs and programmed parameters of the mixed signal section 5.
In addition to voice signaling, the high voltage section provides a substantial gain boost for low voltage signals, and provides both balanced and unbalanced drives for ringing, including multiple wave shapes, such as sinusoidal and trapezoidal signals. The high voltage section is also configured to supply advanced diagnostic information, for application to the low voltage digital signal processing interface. Diagnostic information may relate to tip and ring currents, and operating battery voltage.
The mixed signal section contains low voltage digital communication interface circuitry, including a digital signal processor (DSP) based coder-decoder (codec) . Because the mixed signal section is digitally programmable, the partitioned SLIC architecture of the '976 application is, in effect, a ‘universal’ design, that may be readily programmed to comply with a variety of industry and country telecommunication standards. Programmable line circuit parameters include loop supervision, loop feed, impedance matching and test coverage.
The high voltage analog section 1 includes a receive input unit 10 that interfaces and conditions input voice and ancillary signals, including low voltage signaling and ringing signals, supplied from the codec 6, and couples complementary polarity copies of a voice signal representative current to respective tip and ring amplifier blocks 20T and 20R of a dual mode tip/ring amplifier unit 20. These tip/ring amplifier blocks are selectively biased to operate at a first (close-to-unity) gain for a first signaling mode (off-hook voice signal processing), or at an increased or ‘boosted’ (e.g., ×30 or ×120) gain for ancillary signal processing (e.g., on-hook signaling and ringing). The tip/ring links 2/3 are monitored via a sense amplifier 25.
Reference voltages for the tip/ring amplifier 20 are derived from a battery bias unit 30, which is coupled to the output port 43 of a battery supply switch unit 40. The battery bias unit 30 contains a set of switchable, voltage-divider networks, that are used to selectively bias tip and ring portions of the tip/ring amplifier 20 in accordance with the mode of operation of the SLIC. Battery supply switch 40 selectively couples either a relatively low battery voltage VBL that is applied to a low battery supply switch input port 41, or a relatively high battery voltage VBH that is applied to high battery supply switch input port 42 to one or more output ports (a single one of which is shown at 43, to reduce the complexity of the drawings. The low battery voltage VBL may be on the order of −60 VDC and the high battery voltage VBH may be on the order of −125 VDC. The battery supply switch output port 43 is switchably coupled to tip and ring path voltage divider networks within the battery bias unit 30. The choice of battery voltage depends upon the state of battery supply switch unit 40, whose operation of which is controlled by the mixed signal section 5.
For this purpose, as shown in FIG. 2, the low battery voltage VBL port 41 is coupled through a diode 44 to a node 51 of a battery supply path switch 50. Diode 44 allows transitioning to low battery operation when battery switch is open, or in the event high battery is removed this diode also prevents cross-conduction between VBH and VBL when battery switch 50 is closed. The battery supply path switch 50 is controlled by a battery supply switch control signal from the DSP section 5. (Although only a single set of a diode and battery supply path switch is shown, the battery supply switch unit 40 may contain plural ones of such diodes and associated switches coupled in parallel, for example, a pair of such diode-switch sets, as described in the above-referenced '976 application).
Switch node 51 is coupled to the battery supply switch output port 43, while switch node 52 is coupled to the high battery (VBH) supply switch input port 42. In addition to being coupled to the battery bias unit 30, the battery supply switch output port 43 is coupled to various circuits of the SLIC, such as power transistor circuits.
As shown in FIG. 3, the battery supply path switch 50 may be implemented by means of a Darlington-coupled pair of NPN bipolar transistors Q1-Q2, collectors of which are coupled in common to node 51 (to which the low battery voltage VBL is coupled through diode 44, and which is (default) coupled to output port 43). The emitter of transistor Q2 is coupled to node 42 (to which the high battery voltage VBH is supplied), while the base of transistor Q1 is coupled to receive base drive current from a current source 55.
In order to accommodate whatever current demand may be encountered (including ringing, and other (off-hook) high current requirements) during the various modes of operation of the SLIC, the base bias drive to transistor Q1 may be set at a relatively large current value (e.g., on the order of 50 microamps). This ‘standby’ current drive parameter serves to ensure that the Darlington transistor pair Q1-Q2 will be driven into full saturation and will absorb any current, including those having relatively large values (e.g., currents on the order of 70–100 mA).
Unfortunately, although such a large standby base bias current ensures that the battery supply switch will successfully handle any current demand, it also results in substantial and unnecessary power dissipation (e.g., on the order of 6 mW) during those operational modes where current requirements are relatively low. For example, during on-hook idle mode, in the absence of the need to generate a ringing signal in response to an incoming call, the current demands of the SLIC are relatively minimal, since what is essentially required is to provide just enough current to detect the user going off-hook, in association with the placement of an outgoing call. This reduced current requirement is especially imperative at an installation, such as a remote terminal, where electrical power may be supplied is by a local or back-up battery unit, the power dissipation budget for which is severely constrained.